Wathc for custom tag names in "is" attribute (#6507)
Wathc for custom tag names in "is" attribute (#6507)

Extensions of built-in elements are specified through "is" attribute,
so need to watch for their definition to handle them properly
(loop through child nodes).
alexanderby/darkreader,alexanderby/darkreader,darkreader/darkreader,darkreader/darkreader,darkreader/darkreader,alexanderby/darkreader